Rear projection screen assembly having a projection screen with reflecting rib lenses

ABSTRACT

A projection screen assembly including a projection screen having an illuminated rear side and a light dispersing front side for showing an image with substantially homogeneous luminosity over a wide viewing angle. The light dispersing front side includes a plurality of closely positioned, substantially parallel and, in the use position of the screen, vertically extending lens elements including two inclined reflecting lens crests, and an intermediate lens. The projection screen is characterized by first and second reflecting rib lenses having planes of symmetry inclined toward each other or away from each other, respectively, and a third reflecting rib lens having a plane of symmetry which is normal to the plane of the screen. The use of a plurality of lenses that are approximately identical, but having different inclinations, provide enhanced homogenous luminosity.

The invention relates to a projection screen of the type that isilluminated from behind and which has at its front side light dispersingmeans for receiving beams from an image source with a view to showing animage with substantially homogeneous luminosity over a wide viewingangle. The light dispersing means consist of a number of closelypositioned, substantially parallel and in the application position ofthe screen vertically extending lens elements that have two inclininglens crests, and an intermediate lens.

Such projection screens are used in various apparatuses for generatingan image which is visible to the viewers, eg projection of radar images,aeroplane simulators, television, traffic control lights, microfilmreaders, video games, video monitors with projected image and for theprojection of movies through rear projections. In such apparatuses alight source, placed behind the screen, projects light forwards along aprojection axis towards the screen in preparation for generating, at thelevel of the screen, an image which is spread to all viewers in front ofthe screen.

When a large number of viewers are present, they will normally spreadhorizontally, and thus it is desirable to have a wide dispersion of thelight horizontally over a wide angle. This is the case in particularwith television sets with a rear-projected screen wherein severalviewers are sitting in front of the screen at the horizontal level overa relatively wide angle relative to the screen.

One of the problems encountered in connection with rear projectionsystems is that most of the light is projected along the projection axiswhich means that the intensity of the image increases the closer theviewer is to the projection axis. Colour video devices with rearprojection screens normally use three cathode ray tubes, viz one tubefor each of the primary colours, ie red, green, and blue, which tubesproject the image to the screen through their own projection lens. In aconventional horizontal arrangement of the cathode ray tubes, the greentube is usually positioned centrally on the projection axis, while thered and the blue cathode ray tubes are arranged with their optical axesat an angle of from five to ten degrees with the projection axis of thegreen tube. Unless the screen compensates for these displacedpositionings, a phenomenon called colour shifting will occur. Thisphenomenon expresses itself in that, in case the luminosities of thethree colours are normalised at the centre of the viewer group, theluminosity relationship varies with the angular position in thehorizontal plane all over the viewing angle. This implies that aviewer's perception of the image depends on his place in the horizontalplane in front of the screen.

Furthermore, when rear projected screens are used in and exposed toambient light, the contrast of the projected image is affected by thelight reflection on the front side of the screen. Thus, it is desirableto reduce the reflection of ambient light from the front side of thescreen. Various masking technologies have been suggested for thereduction of light reflection, wherein a black, non-reflective sheet hasbeen inserted between the lenses, or the entire front of the screenwithout black stripes has been dulled.

Various rear-projected screens have previously been suggested for thepurpose of increasing the viewing angle in the horizontal plane. U.S.Pat. Nos 4,418,986, 4,469,402, 5,428,476, and 4,509,822 disclose such asystem where a screen is used which has a rear sheet like a Fresnellens, which is able to collimate the beams from the image source inparallel beams, and a front sheet which is configured with a dispersionlens with vertical, continuous ribs/tops for distributing the light overa specified horizontal viewing angle. According to the well-knowntechnique, the front side of the screen is in essence divided into twolens types; a cylindrical lens for dispersal of the light for a narrow,forwardly directed field of vision, and a total-reflecting lens forfurther increasing the viewing angle.

Among experts it is well known that it is technically difficult toaccomplish a homogeneous light dispersion when using the well-known lensconstructions since the well-known systems are constructed with one ortwo lenses —one lens that transmits the light within a narrow horizontalviewing area, eg +/−25°, and a lens with inner total-reflection thatspreads the light from +/−25-30° up to +/−60°, respectively. It appliesto both lens types that it is technically difficult to accomplishsufficient overlapping between the two lens types, and especially whenthe rear projection screen is used for video projection where usuallythree projection cathode ray tubes are used—one cathode tube for eachfundamental colour, and where the optical axes of these usually have amutual angle of 7-12°.

If eg the green image is projected at right angles on the rear side ofthe screen, the blue and red images shall be projected against the rearside of the screen with an inclination of eg the mentioned 7° relativeto the normal of the screen. However, this may have the effect that aperson watching the screen diagonally from the front will see an imagewhich is dominated by either blue or red—depending on whether therelevant person is closest to the optical axis of the projector emittingthe blue image or to the projector emitting the red image. In thefollowing, this colour distortion will be designated ‘colour-shading’.

The rear projection screen according to the present invention ischaracterized in that two lens crests are used whose axes of symmetryincline toward each other or away from each other, respectively, andwhose axes of symmetry further form an angle relative to the normal tothe plane of the screen, and wherein a further lens has been insertedbetween two said lenses, the axis of symmetry of which is normal to theplane of the screen. The use of a plurality of lenses that areapproximately identical, but having different inclinations, provides amore homogenous luminosity.

Thus, the light coming from behind will be reflected one hundred percentwhen it hits the inner side of two symmetrical lens crests since theprecondition for the total reflection is present. A light beam that hitsthe inner side of the left lens part in this way will be radiatedthrough the lens top. In this way, through the differences in lensinclinations in combination with the inserted, transmitted side lensesat the foot of the inclining lens crests, it is accomplished that thelight is dispersed across a wider fan despite the lenses beingapproximately identical. It also follows that the colour-shadingexplained above will be neutralised as a consequence of thetotal-reflected light radiated through the reflecting lenses beingspread in a arc of an angle which is approximately 10-30 percent widerthan the angle which the optical axes of the light source form incombination.

A viewer viewing the image diagonally from the front will thus perceivethree merged images since the lens crests and the transmitted lenseswill deflect the light in such a way that beams from the three lightsources will be perceived as being approximately parallel. At the sametime the advantage will be accomplished which is associated with lenseswith one hundred percent reflecting sides; viz the image can be viewedunder a wide side-viewing angle. In order to ensure that also theintermediate lenses, where the forwardly directed light is radiatedthrough the tops are blended in such a way that the colour-shadingproblem is solved, the lenses are made with different curvatures. Thismeans that the relation between the volume of light that is radiatedthrough the asymmetrical lens tops may vary within wide ranges inaccordance with the severity of the colour shading problem in theparticular appliance.

The invention will now be described in further detail with reference tothe drawings, wherein

FIG. 1 is a sectional view of a video device;

FIG. 2 shows a colour projection TV system comprising a projectionscreen and three cathode ray tubes with associated lenses;

FIG. 3 is a perspective, sectional view of the screen according to thepresent invention;

FIG. 4 is a sectional view through the Figure shown in FIG. 3 accordingto the present invention for illustrating the path of beams;

FIG. 5 is a view of the present invention in which, however, theintermediate, transmitting lenses are planar;

FIG. 6 illustrates an alternative embodiment wherein the intermediatelenses are convex;

FIG. 7 illustrates a further embodiment wherein the intermediate lensesare concave;

FIGS. 8a and 8 b illustrate a figure with reference measurements andangles for the examples mentioned in the description;

FIG. 9 shows light transmission curves for the total-reflecting lensesand for the transmitting lenses, respectively, as a function of theviewing angle according to the present invention;

FIG. 10 illustrates optical measurement results of a test according tothe present invention; and

FIG. 11 shows another embodiment of the present invention.

Prior to describing the embodiments of the present invention, thegeneral design of a projection system using rear projection screensaccording to the invention will be explained with particular referenceto FIGS. 1 and 2. FIG. 1 shows as an example a sectional view of aprojection TV-set. The video projection device 1 is often constructedwith three cathode ray tubes 2, a lens 4, a mechanical coupling 3, amirror 5, as well as a rear projection screen 6.

FIG. 2 shows a rear projection screen with three light sources that havea surface provided with lenses for the illumination and application ofprojection screens of the kind discussed herein. The three projectors 7,8 and 9 project TV-images against the rear side of a projection screen23. Each projector emits green, red and blue light, respectively. Thethree projectors are arranged horizontally side by side in such a waythat the optical axis of the projector 8, which is most often the greenprojector, is perpendicular to the screen 23.

The optical axes 13,15 of the projectors 7,9 often form an angle of 8-12degrees relative to the projector 8.

By means of the three projectors 7, 8 and 9 aided by the lenses 10, 11and 12 mounted in front, it is possible to form an image which isenlarged relative to the projectors, on the screen 23. Very often, aprojection screen 23 has a lens structure at the rear 16 and at thefront 17. The rear side 16 is often configured as a Fresnel lens thatconverts the divergent light beams into parallel light beams. The lightbeams that are transmitted parallel through the rear projection screen23 medium are dispersed from the front surface of a lens structure 117in a suitable viewing angle.

In the following, preferred embodiments of the invention will bedescribed with reference to the drawings, wherein:

FIG. 3 are perspective views of rear projection screens according to theinvention. As will appear from this figure, the rear projection screenaccording to the invention has on the surface of its image side asurface 34 provided with lenses that extend vertically in the in-useposition of the screen.

In the figures, 18 is used to designate symmetrical, total-reflectinglens with the flanks 19,20 that, in turn, abut on each theirtransmitting lens 21,22 arranged at the foot of the lenses 18. The axesof symmetry of the two lenses 26,27 form an angle ψ relative to thenormal with the screen. The rear side 24 shows a section of a Fresnellens structure for parallellising the light emitted from the lightsource 25.

FIG. 4 shows a section of FIG. 3 according to the present invention forillustrating the beam path. For clarity reasons, only the beam path ofthe green light source is shown. As it appears from the figure, a lightbeam 29 from the green light source will pass through the centre of thescreen lens 28 without deflection. At the arc-shaped part of the lens28, the light will be deflected within a small visual range, eg +/−15°.

The present invention differs substantially from the prior arttechniques in that the total-reflecting lenses 26 and 27 are configuredsuch that their axes of symmetry are not parallel, as will also appearfrom study of the light beams 30,30′ and 32,32′ and 33,33′, and herebythe previously mentioned colour-shading problem is neutralised.

In FIG. 9 relative measurement results are given for respective curves42 of the right lens 27, and the left lens 26 and the lens 18. Thebell-shaped curve 43 in the middle is the total measurement result forthe lenses 40, 39 and 28 in FIG. 4.

FIG. 10 shows all lenses in operation for horizontal and verticalmeasuring, respectively, in accordance with the figure shown in FIG. 4.From the figure, it will appear that the screen according to theinvention showed a very constant light at the horizontal level within aviewing angle of +/−45 degrees, curve 46. In order to also accomplish acertain vertical light dispersion, curve 47, light refractive particlescan be added to the screen without changing the remaining properties ofsame. The quantity of such light scattering particles should be amaximum of 45 g/m², when these particles have an optical refractionindex within the range of from 1.5 to 1.58 and a grain size of from 3 μmto 65 μm.

The, bell-shaped curve 44 shown in FIG. 10 is obtained only when a glasspowder is added. It should be noted that usually the viewers arepositioned in the horizontal plane, and thus the constructor endeavoursto achieve the widest viewing angle at that level.

What is claimed is:
 1. A rear projection screen assembly comprising: animage source for projecting light image beams; a projection screenhaving a rear side and a front side, the rear side facing the imagesource, the front side comprising a light dispersing material forreceiving beams from the image source and for deflecting the individualbeams to project an image with a substantially homogeneous luminosityover a wide viewing angle; said light dispersing material comprising aplurality of closely arranged, substantially parallel, longitudinallyextending reflecting rib lens elements, each having a plane of symmetryand two lateral surface portions; the lateral surface portions of thereflecting rib lenses being total-reflecting with respect to light fromthe image source, the central portions of the reflecting rib lensesbeing light transmitting, and the lateral surface portions ofneighboring reflecting rib lenses being adjoined by transmitting lenses;and wherein the reflecting rib lenses comprise: first reflecting riblenses having a plane of symmetry inclined at an angle ψ with respect toa plane normal to the screen, second reflecting rib lenses having aplane of symmetry inclined at an angle −ψ with respect to a plane normalto the screen, third reflecting rib lenses having a plane of symmetrynormal to the screen, and wherein ψ differs from 0°.
 2. The rearprojection screen assembly according to claim 1, wherein thetransmitting lenses are either planar, convex or concave.
 3. The rearprojection screen assembly according to claim 1, wherein the projectionscreen is made of optical plastic materials.
 4. The rear projectionscreen assembly according to claim 3, wherein the optical plasticmaterials are acrylic or a mixture of acrylic and styrene.
 5. The rearprojection screen assembly according to any one of claims 1, 2, 3 or 4,wherein the rear side of the projection screen has a Fresnel lens, orwherein the screen comprises a rib-lens sheet portion, and a Fresnelsheet portion.
 6. The rear projection screen assembly according to claim1, wherein the projection screen is laminated.
 7. The rear projectionscreen assembly according to claim 1, including in the screen, alight-dispersing medium of an organic or inorganic pigment or of amixture of organic and inorganic pigment.
 8. The rear projection screenassembly according to claim 1, wherein the rear side of the projectionscreen comprises horizontal, parallel extending rib lenses fordispersion of the light in the vertical direction during use.
 9. Therear projection screen assembly according to claim 1, wherein tops ofthe reflecting rib lenses are dulled to avoid reflections from thesurroundings.